1. Field of the Invention
The present invention relates generally to the fabrication of high grade epitaxial layers and, more particularly, to a process of making strain-free, heavily carbon-doped III-V epitaxial layers and products so made.
2. The Prior Art
Semiconductors (also known as semiinsulators), in particular compound semiconductors, are formed in a variety of ways. One preferred way involves the metalorganic vapor phase epitaxy (MOVPE) based growing of semiconductors. Another preferred way involves the deposition of the semiconductor layers by molecular beam epitaxy (MBE). As known, a semiconductor is a solid crystalline material whose electrical conductivity is intermediate between that of a metal and an insulator and ranges from about 10.sup.5 mhos to about 10.sup.-7 mho per meter. A semiconductor usually is strongly temperature-dependent.
The several layers of a semiconductor device are either n-type or p-type, depending on their doping during deposition. A semiconductor device is one in which the characteristic distinguishing electronic conduction takes place within a semiconductor, that is the solid semiinsulating crystalline material. The n-type semiconductor layers are typically doped during their deposition with a dopant such as silicon (Si) or sulfur (S) in the range of about 0.1 to about 100 parts-permillion (ppm). A wide variety of p-type dopants have been used in the MOVPE based growth of semiconductors. The most common dopant elements include Zinc (Zn), magnesium (Mg) and beryllium (Be). However, the control over the dopant concentration and the dopant profile using one of these dopants has been difficult due to gas system "memory" effects and their relatively high diffusion coefficients. See M. L. Timmons et al, "An Alternative Mg Precursor For p-Type Doping of OMVPE Grown Material," Journal of Crystal Growth 77 (1986) pp. 37-41. See also W. S. Hobson et al, "Redistribution of Zn in GaAs-AlGaAs Heterojunction Bipolar Transistor Structures," Applied Physics Letters, 56 (1980) pp. 1251-1253. Workers in the field have, accordingly, investigated other p-type dopants, including carbon. Initially, the intentional introduction of carbon as a p-type dopant in epitaxy from a wide variety of hydrocarbon sources had been less than successful due to the low carbon content levels that could be achieved despite a wide variety and range of growth conditions employed. Toward the end of the 1980's, a group of workers have succeeded in achieving high levels of active carbon by using trimethylgallium [Ga(CH.sub.3).sub.3 ] (TMGa) and arsine, AsH.sub.3, in metalorganic molecular beam epitaxy (MOMBE). Others then succeeded in the controll-ed doping of GaAs layers in MOVPE growth through the combined use of trimethylarsenic [As(CH.sub.3).sub.3 ] (TMAs) and arsine, AsH.sub.3 and also by selecting the proper growth temperature during the TMGa-based growth of GaAs. See T. F. Kuech, et al., "Controlled Carbon Doping of GaAs by Metalorganic Vapor Phase Epitaxy," Appl. Phys. Lett. 53(14), 3 October 1988, pp. 1317-1319. See also P.M. Enquist, "P-Type Doping Limit of Carbon in Organometallic Vapor Phase Epitaxial Growth of GaAs Using Carbon Tetrachloride," Appl. Phys. Lett. 57(22), 26 November 1990, pp. 2348-2350; and G. B. Stringfellow et al, "Carbon in Molecular Beam Epitaxial GaAs," Appl Phys Lett 38 (3), 1 February 1981, pp. 156-157.
Using carbon as p-type dopant, the doping profile can be made extremely sharp. Further, since carbon in Ga As evinces a diffusion coefficient that is much lower than that of either Zn or Mg, carbon-doped GaAs layers have been found suitable for use as p.sup.- -channel stop layers and also for high doping applications in high performance electronic devices. Such high performance electronic devices include super lattice structures, tunnel diodes, laser diodes and heterojunction biopolar transistors (HBTs). HBTs are currently being investigated by some workers in the field as potential replacements for field effect transistors (FETs) in certain high power, low noise applications due to their vertical geometry, higher individual layer conductivity and their proven lower 1/f noise.
In the epitaxial forming of GaAs layers, carbon is incorporated as a substitutional acceptor on the column V sublattice and is covalently bonded to four surrounding column III atoms. Since the typical Ga-C binding energy and bond length are slightly different than those for Ga-As, the substitution of carbon in the lattice results in a slight distortion of the crystal lattice structure. As the carbon concentration increases, so does the distortion and the macroscopic strain on the layer. At some point, the strain will become so great that it overcomes the shear strength of the material itself and begins to introduce native crystal defects, having a detrimental effect on device performance. A strengthening of GaAs single crystals by the substitution of a few percent indium on Ga sites has been observed and reported. See H. Ehrenreich et al, "Mechanism For Dislocation Density Reduction in GaAs Crystals by Indium Addition," Appl Phys Lett 46 (7), 1 Apr. 1985, pp. 668-670. And in the liquid encapsulation Czochralski (LEC) growth of GaAs, the addition of 1% indium has been reported to reduce the dislocation density in the resultant material. See G. M. Blom et al., "Effect of Iso-Electronic Dopants on the Dislocation Density of GaAs," Journal of Electronic Materials, Vol. 17 No. 5, 1988, pp. 391-396. No one to date has however so far addressed the balancing of distortion and strain of a heavily carbon-doped p-type epitaxial layer caused by the incorporation of carbon atoms therein.